gistfile1.txt

Alright, so let's say you want to 'bucket fill' at some coordinate (x,y)

Let's pretend that the pixels at the coordinate look like this

    123
    456
    789

That is, you have (x, y) pointing to the pixel '5' in this case.
Assuming that (0,0) is the top-left corner, then the pixel '1' is at
(x - 1, y - 1) and similarly, the pixel '9' is at (x + 1, y + 1).

So if you're given a coordinate (x, y) and you want to enumerate the pixels
around that coordinate(or rather, generate the coordinates which you then
use to paint the pixels afterwards) your haskell code might look something
like this:

getAdjacent (x, y) =
    [(x', y') | x' <- [x - 1 .. x + 1], y' <- [y - 1 .. y + 1]]

λ: let getAdjacent (x, y) =     [(x', y') | x' <- [x - 1 .. x + 1], y' <- [y - 1 .. y + 1]]
λ: getAdjacent (3, 3)
[(2,2),(2,3),(2,4),(3,2),(3,3),(3,4),(4,2),(4,3),(4,4)]

This is from ghci, so it works. But there are corner cases. If you ask for
the pixels around (0, 0), you get coordinates like (-1, -1) which aren't
valid. This is easy to get around, but it requires your function to know
about the dimensions of the picture.

getadjacent (x, y) dimX dimY =
    let coords = [(x', y') | x' <- [x - 1 .. x + 1], y' <- [y - 1 .. y + 1]]
    in filter validCoord coords
  where
    validCoord (i, j) = i >= 0 && i < dimX && j >= 0 && j < dimY

Note that this function is just an example.. I have little time so it might
not type check.
Additionally, since you're filtering colors and so on, you could add another
function that checks if the color is the same as color at the origin. This
assumes you have some way of accessing the canvas and so on, so you might
have to add the canvas as an argument(or use the State monad or some such).

getadjacent (x, y) dimX dimY canvas =
    let coords = [(x', y') | x' <- [x - 1 .. x + 1], y' <- [y - 1 .. y + 1]]
    in filter validCoord coords
  where
    validCoord (i, j) = i >= 0 && i < dimX && j >= 0 && j < dimY
    validColor (i, j) = colorAt (i, j) canvas == colorAt (x, y) canvas
    colorAt = -- some definition for colorAt that gets the color at a coordinate.